A magnetic resonance imaging (MRI) apparatus is a diagnostic imaging apparatus for medical use, primarily utilizing nuclear magnetic resonance phenomena of proton. This apparatus applies a radio frequency pulse to a subject placed in a static magnetic field, exciting nuclear magnetization, and measures a magnetic resonance signal. On this occasion, application of a gradient magnetic field provides positional information, and creates a image. The MRI apparatus sets up no limitations on a portion to be imaged, allowing any cross section to be imaged noninvasively.
In general, while applying a slice gradient magnetic field which specifies a plane on which a tomographic image of the subject is to be obtained, the MRI apparatus simultaneously provides an excitation pulse in order to excite magnetizations in the plane. Accordingly, nuclear magnetic resonance signals (echoes) are obtained, which are generated in the course of precession of the magnetization excited by the pulses. In order to provide the magnetization with positional information, the MRI apparatus applies a phase-encoding gradient magnetic field and a readout gradient magnetic field, which are perpendicular with each other within the tomographic plane, during the time from the excitation to the echo acquisition. Then, the echoes being measured are arranged in a k-space which defines its horizontal axis as “kx” and the vertical axis as “ky”. One echo occupies one line which is parallel to the kx-axis. The k-space is inverse Fourier transformed to reconstruct an image.
The pulse and each of the gradient magnetic fields for producing the echo are applied according to a predetermined pulse sequence. There are known different types of pulse sequences for different purposes. By way of example, a gradient echo (GrE) type high-speed imaging method is a method which repeatedly executes its pulse sequence, and allows the phase-encoding gradient magnetic field to vary sequentially for every repetition to sequentially measure the echoes in a number required for obtaining one tomographic image.
One example of this GrE-type pulse sequence is a phase compensation type pulse sequence. In this pulse sequence, a gradient magnetic field pulse is added to GrE so as to bring zero to a time integration value of the gradient magnetic field of each axis. A degree of a flip angle of the radio frequency (RF) magnetic field pulse is generally larger than that used in the other GrE type pulse sequences, and the phase thereof is inverted alternately. In addition, the repetition time (TR) is shorter and it is around 5 ms.
The GrE type imaging method as described above repeatedly excites magnetization, before executing a pulse sequence for measuring echoes required for reconstructing an image (imaging mode), in order to obtain a steady state of magnetization. This is referred to as anon-imaging mode. In the non-imaging mode, the same pulse sequence as that used in the imaging mode is executed for a given number of times without measuring echoes. In many cases, however, in order to shift the magnetization to the steady state with less times of execution, the flip angle of the RF pulse in the non-imaging mode may be gradually increased from a small angle and made closer to the angle used in the imaging mode.
Moreover, in the high-speed imaging methods as described above, the flip angle greatly influences imaging contrast. Therefore, an angle providing a particular image contrast is chosen typically from the range of 1 to 90 degrees as the flip angle for the imaging mode, and the flip angle is not usually changed during the imaging mode.
Such GrE-type high speed imaging methods as described above are frequently used for clinically conducting cardiac diagnosis, vascular diagnosis on the thoracicoabdominal part, or the like. In the case of taking an image of the heart, there is widely employed a method which enhances time resolution of the imaging by using the ECG (electrocardiogram) gating, since the cardiac cycle is short, i.e., approximately one second. In other words, this method changes the phase encoding in sync with triggering of R wave in an electrocardiogram, and measures echoes required for reconstructing one image across multiple heart beats. Breathing during the imaging causes a body motion, and this may generate a ghost in the reconstructed image. Therefore, it is general to conduct the imaging during breath-hold. When taking an image of the heart, the information as to the movement during the cardiac cycle is important, and therefore moving images (cine images) are frequently taken.
A part targeted for imaging, such as the thoracicoabdominal part which is influenced by body motion caused by breathing, is typically subjected to the imaging with breath-hold. If it is not possible to take all the images while holding the breath, measurement of echoes the number of which is necessary for reconstructing an image is divided into multiple imaging times, and the images are taken by repeating the breath-hold and the measurement. However, such imaging that repeats the breath-hold as described above may place burden on a patient, and therefore, there is a respiratory gated method which takes an image while monitoring the state of breathing under the condition of free breathing. As a method for monitoring the respiration, following methods may be employed; a method using an external device for directly measuring the respiration state, a method for incorporating another imaging for measuring the respiration state into the image taking (e.g., see the patent document 1).
On the other hand, in MRI, a magnetic resonance frequency becomes higher in proportion to the magnetic field intensity. In this connection, there arises a problem of increase in absorption of RF electric power into human bodies, called specific absorption rate (SAR), and development of countermeasure against this problem constitutes a subject of researches. The SAR indicates RF irradiation power per unit time, and it is proportional to the time integration value of square of the flip angle. A reference value of the maximum SAR for total human body is defined to be 4 W/kg. When a GrE type pulse sequence is used, the RF irradiation is repeated in a short period time, and therefore the SAR becomes large. In particular, phase compensation type GrE pulse sequences use a short TR and a large flip angle. Therefore, it is difficult to apply such sequence to a human body in a high magnetic field apparatus using a magnetic field of about 3 Tesla or more in view of safety. By way of example, for the case that a phase compensation type GrE pulse sequence using the flip angle of 60 degrees and TR of 3 ms is executed in an apparatus using the magnetic field of 3 Tesla, the SAR is calculated to be 4.7 W/kg. This value exceeds the reference value, and therefore it is impossible to perform imaging.
In order to reduce SAR, it is necessary to prolong the repetition time TR or to make the flip angle smaller. However, it is not preferable to extend TR since this may cause extension of the imaging time. On the other hand, if the flip angle is made smaller, it may degrade contrast and an S/N ratio.
To solve this problem, in consideration of the specific absorption rate SAR, there has been proposed a method of changing the flip angle of RF excitation pulse for the imaging mode according to the amount of phase encoding so that the S/N ratio should not be lowered (Patent document 2). This method is based on the fact that the S/N ratio in MRI is generally determined by the S/N ratio of echoes having a small phase encoding amount, and maximizes the flip angle when an absolute value of the phase encoding amount is minimum, whereas minimizes the flip angle when an absolute value of the phase encoding amount is maximum, so that the S/N ratio should not be reduced even when the flip angle is changed.